1. Field of the Invention
The present invention relates to a slurry distribution system for a chemical-mechanical polishing (CMP) process performed during the fabrication of a semiconductor device, and more particularly, to a system for distributing slurry in which the slurry supply line moves up and down, such that the lower inlet point of the supply line is maintained at a predetermined distance below the top surface of the slurry in the slurry tank.
2. Background of the Related Art
Unevenness on the surface of a wafer is a serious problem, due to the high-integration and multiple layer structure of the circuit distribution of semiconductor devices. Therefore, in order to planarize or flatten the uneven wafer surface, leveling materials or methods such as SOG (Spin On Glass), Etch Back, Reflow, etc., were developed. However, these methods still do not completely remove the unevenness, and accordingly, such methods have given way to current chemical-mechanical polishing (CMP) techniques.
The CMP technology levels the surface of the wafer using chemical and physical reactions, wherein the wafer comes into contact with the surface of an elastic pad, and slurry is supplied so as to react chemically with the wafer surface. Therefore, relative movement between the polishing platen and wafer holder physically levels the uneven wafer surface.
Removal rates and uniformity are important criteria for effective CMP technology. Variables such as the processing conditions of the CMP equipment, the kind of slurry, and the kind of carriers, etc., greatly affect both the removal rate and uniformity. The pH of the slurry and its ion concentration also have an impact on the planarization process. The slurry can be one of two kinds, one for metal and one for oxide, and the slurry is stored in a slurry distribution system to be supplied during the planarization process.
FIG. 1 shows a conventional slurry distribution system for supplying the slurry into the CMP equipment, comprising a tank 11 for storing a certain amount of slurry, and a slurry supply line 12 penetrating through the top side of the tank 11 for supplying the slurry into the CMP equipment (not shown).
The slurry is supplied through the slurry supply line 12 into the CMP equipment, and is conventionally circulated by using either a pump, compressed nitrogen, or vacuum methods. The height H from a lower opening 12a of the slurry supply line 12 to the bottom of the tank 11 is generally less than 5 cm.
A return line 14 is installed penetrating the top of the tank 11, through which any excess slurry supplied to the CMP equipment, but not needed there, is returned to the tank 11. A supplementary line 15 from a slurry mixing tank (not shown) is connected to the return line 14 so as to additionally supply new slurry when necessary.
A plurality of level sensors are installed inside the tank 11 so that a certain amount of slurry is always kept therein. For example, four level sensors 16, 17, 18, 19 are installed one by one from the upper side to the lower side of the tank 11 so as to check the amount of slurry remaining inside the tank 11. When slurry is exhausted down to the level of the third or the fourth level sensor 18 or 19, new slurry is supplied through the supplementary line 15 and return line 14 until the top surface of the slurry reaches the level of the first or the second level sensors 16 or 17.
Generally, the empty volume between the top inside surface of the tank 11 and the top surface of the slurry in the tank is filled with air or nitrogen. However, in the above slurry distribution system, slurry residue, which becomes hardened and lumped due to the distortion of the slurry, forms on the bottom of the tank 11. When the distorted and lumped slurry residue enters the CMP equipment through the slurry supply line 11 according to the flow movement inside the tank 11, it causes fine scratches on the wafers while performing the CMP process.
These scratches cause malfunctioning of the semiconductor devices and deterioration of their qualities. This kind of problem is more serious with highly integrated semiconductor devices.
In order to prevent the distorted and lumped slurry residue from entering into the CMP equipment, filters are installed in the slurry distribution system or supply pipes of the CMP equipment, but scratches are not completely prevented because the slurry residue also shortens the life of the filters.
The distorted and lumped slurry residue can be found in many areas of the slurry supply system, including the return line 14, the inside wall of the tank 11, etc., and is more thickly distributed closer to the bottom of the tank 11 because of its weight.
As shown in FIG. 1, the lower opening 12a of the slurry supply line 12 is very close to the bottom of the tank 11 in order to supply as much of the slurry in the tank 11 to the CMP equipment as possible. The distance H between the opening 12a of the slurry supply line 12 and the bottom of the tank 11 is generally less than 5 cm, and therefore, the distorted and lumped slurry residue on the bottom of the tank 11 enters the supply line 12 and proceeds into the CMP equipment according to the slurry flow path in the system.